online at www.sciencedirect.com SCII[NCE
FEASIBILITY STUDY OF THE 0 II 83.4~nmIMAGING OF THE IONOSPHERE AND MAGNETOSPHERE A. Yamazaki’, I. Yoshikawa’, Y. Takizawa3, W. Miyake’, and M. Nakamura’ ‘Communications Research Laboratory, 4-2-l Nukuikita-machi, Koganei, Tokyo 184-8795, JAPAN 21nstitute of Space and Astronautical Science, 3-l-l Yoshinodai, Sagamihara, Kanagawa 229-8510, JAPAN ‘RIKEN, 2-1, Hirosawa, Wakq Saitama 351-0198, JAPAN
ABSTRACT Recent in-situ plasma observations find that large amounts of O+ are escaping from the terrestrial ionosphere to the magnetosphere. Remote-sensing methods using the extreme ultraviolet (EUV) emission of O+ have been expected to be a powerful tool to provide a global perspective on the escaping processes. The overall picture is also very important for the practical use such as monitoring space weather. O+ ions resonantly scatter the solar photons with wavelength 83.4 nm. The key to the successof the observation is to prevent from detecting the H Ly-a line (121.6 nm), which is stronger than the predicted 0 II emission by four orders of magnitude. We have successfully detected 0 B emission from the uppermost part of the ionosphere using the sounding rocket SS-520-2 to investigate heavy ion escape from the, cusp/cleft region. This successdemonstrates the capability of the remotesensing method to take an instantaneous 2-dimensional image of the O+ distribution, and provides a way for optical observation of the magnetosphere. We plan to obtain 0 II images of the polar wind using the Telescope for Extreme ultraviolet light, which is an upgrade version of the instrument for the sounding rocket, in the Upper atmosphere and Plasma Imager component (UPI-TEX) on the SELenological and ENgineering Explorer (SELENE). We refer to the feasibility of the 0 II imagery from the lunar orbit satellite. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. INTRODUCTION Standard theories on the polar wind indicated that only light ions such as I-F and He’ could overcome the terrestrial gravitational potential to escape from the polar ionosphere due to pressure gradients along open field lines and that the 0’ outflow was limited due to both its large mass and its loss in charge exchange with neutral H in the atmosphere [e.g., Axford, 1968; Banks and Holzer, 19691. Observations by polar orbiting satellites such as Dynamic Explorer 1 (DE-l) and Akebono (EXOS-D), however, found that the upward flux of 0’ was comparable to that of H, especially under active geomagnetic conditions and during periods of a high solar activity [e.g., Chandler et al., 1991; Abe et al., 1993, 19961. Furthermore, Geotail observed a cold dense 0’ flow of ionospheric origin in the distant tail lobe [Seki et al., 1996, 1999, and therein references]. The difference between the rate of 0’ outflow from the polar ionosphere and that of O+ escape from the magnetosphere to interplanetary space indicates that there should be unknown transport processes for O+ in the magnetosphere [Seki et al., 20011. The 2dimensional (2-D) 0 II imagery, which provides a global 0’ distribution, is expected to identify the transport routes and mechanisms of the cold 0’ ions and to reveal a quantitative balance between the supply and loss including the thermal plasma. Remote-sensing methods using the extreme ultraviolet (EUV) emission of He’ (He II 30.4 nm) and O+ (0 II 83.4 nm) have been expected to be a powerful tool to provide global perspectives on the escaping processes [Chiu et al., 1986; Meier, 1991; Williams et al., 19921, because conventional in-situ plasma particle measurement methods cannot detect plasma particles far from the satellite or thermal plasmas. The fundamental technology to detect the EUV emission began with the He II emission through the previous rocket experiments. The 2-D He II Adv. Space Res. Vol. 32, No. 3, pp. 441-446,2003 8 2003 COSPAR. Published by Elsevier Ltd. All rights Printed in Great Britain 0273-l 177/$30.00 + 0.00
imaging of the terrestrial plasmasphere from its outside was performed by both the Planet-B (Nozomi) spacecraft [Nakamura et al., 2000; Yoshikawa et al., 20OOa, 20011 and the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission [Burch et al., 2001a, 2001b; Sandel et al., 20011. Cold He’ ions in the near-Earth plasma sheet were also optically detected by Planet-B [Yoshikawa et al., 2OOOb]. Optical observations of oxygen ions around the Earth have not yet been performed, because of the difficulty to get rid of the bright H Ly-a line from the geocorona. We had developed a primitive equipment named the extreme Ultraviolet (XUV) sensor that has a thick indium film in the band pass filter. It has enough a high efficiency ratio of the 0 II emission to the H Ly-a line [Yamazaki et al., 20021. On December 4, 2000, it flew onboard the sounding rocket SS-520-2 to investigate the mechanism of ion acceleration and/or heating at the cusp/cleft region. The XUV sensor successfully detected 0 II emission from the uppermost part of the polar ionosphere. The altitudinal variation from 150 through 1100 km indicated the existence of 0’ ions beyond the polar ionosphere and suggested that 0’ ions that are energized in the cusp/cleft region may drift to the uppermost part of the polar ionosphere. Using an upgraded version of this instrument we plan to obtain the 0 Il images of the polar wind from the lunar orbit with the SELenological and ENgineering Explorer (SELENE) [http://moon.nasda.go.jp/]. The SELENE will be launched by the H-IL4 rocket in 2005 to be put into orbit around the moon. The SELENE project will carry out scientific observations of the moon, at the moon, and from the moon. The Upper atmosphere and Plasma Imager (UPI) on SELENE takes 2-D visible and extreme ultraviolet images of atomic and plasma distributions in the upper atmosphere and ionosphere and around the Earth. The component has two telescopes; one is a Telescope for VISible light (UPI-TVIS), and the other is a Telescope for Extreme ultraviolet light (UPI-TEX). The UPITVIS imager detects the four emission lines (427.8, 557.7, 589.3,630.0 nm) to simultaneously take aurora1 images around both of Earth’s polar regions, and the UPI-TEX imager detects the resonance scattering emission of oxygen ions (0 II: 83.4 nm) to take images of 0’ escape from the polar ionosphere. The simultaneous observation of these telescopes reveals the relation between the precipitation of aurora1 particles and the outflowing ions. This paper covers the design of the UPI-TEX imager and calculates the S/N ratio in order to estimate the spatial and temporal resolution from 0 II intensity of the polar wind simulated by the previous statistical analysis of ion outflow. We also discuss the scientific topics for the 0 II imagery. SCIENCE TARGET The UPI-TEX imager detects the 0 Il emission scattered by 0’ ions of the ground state around the Earth, especially in the thermal energy range because of the large emission rate factor. The observational targets are the polar ionosphere, the polar wind, and the inner magnetosphere. Oxygen Ion Outflow from Polar Ionosphere Previous studies reported that the occurrence of the 0’ outflow event depends on the direction of the interplanetary magnetic field (IMF), the geomagnetic activity, the seasonal condition, and the solar activity [e.g., Chandler et al., 1991; Abe et al., 1993, 19961. Recently the Low-Energy Neutral Atom imager (LENA) on IMAGE observed the oxygen ion burst outflow in response to an enhancement of the dynamic pressure of the solar wind at the moment of the passage of a shock produced by a coronal mass ejection [Moore et al., 2001; Fuselier et al., 20011. These observations indicated that the O+ outflow flux has a long and short-term variation, and a constant monitoring and simultaneous observation of solar wind plasma are necessary to identify 0’ outflow mechanisms. The LENA imager can detect the energetic neutral atoms with the energy range of lo-750 eV [Moore et al., 20001. The UPI-TEX imager, however, can detect the 0 II emission scattering by the thermal 0’ as well as the low energy ions, i.e., it can measure O+ with thermal energy range where ions will return to the atmosphere due to the gravitational force once they flow upward. The UPI-TEX imager observes at an isometric orbit from the Earth to detect the short-term variation more easily than the IMAGE mission. Transport Route of Oxygen Ions from Ionosphere into Magnetosphere Chappel et al.  reported that the plasma outflow from the ionosphere has enough flux to explain the plasma density in the plasma sheet. Seki et al. [1996, 20001 found that the cold 0’ beam exists in the lobe/mantle region of the magnetotail and that the 0’ energy acquisition cannot be explained if 0’ ions directly flow from the polar ionosphere, and they suggested other transport route for oxygen ions. But the route escaping from the ionosphere to the magnetosphere has been not yet observed, because the in-situ measurements cannot detect
plasma far from the satellite. The optical observation can record a 2-D snapshot of the comprehensive plasma distribution to produce a significant achievement of an unknown escape route. Thermal Oxygen Ion Distribution in Magnetosphere Thermal 0’ observation is not performed in the inner magnetosphere, because the existence of the high energetic particles in the radiation belt contributes the large noise count level. The high-energy electrons at the outer radiation belt injected inward during the geomagnetic storm [Obara et al., 2000], and the thermal plasma should inject inward together and should precipitate to the ionosphere at the low latitude. Furthermore the oxygen ions flux between supply and loss in the magnetosphere does not keep in balance [Seki et.al., 20011. We suggest that these problems arise from no observation of the thermal plasma distribution in the high-energy plasma environments of the magnetosphere. If the 0 II optical observation is successfully performed, the distribution of the thermal 0’ is constantly obtained, and its behaviour and its quantity balance will be cleared. FEASIBILITY
OF 011 IMAGERY
OF POLAR WIND
The 0 II emission intensity is estimated from the previous statistical study of thermal ion outflow measured by Akebono [Abe et al., 19931. We exhibit the key point for the 0 II optical observation, and show the design of the UPI-TEX imager for the meaningful 0 II observation. We also calculate the signal-to-noise (S/N) ratio to estimate the extent to distinguish between a spatial variation and a temporal transition for the UPI-TEX imager. Estimation of 0 II Intksity The Akebono measurements of thermal H+ and 0’ in the high-altitude polar ionosphere (6000-9000 km) are used to statistically analyze the outflow flux normalized to 2000&n altitude in the four magnetic local time sectors (21-03, 03-09, 09-15, and 15-21 MLT) [Abe et al., 19961. The dependences of the outflow flux on the IMF direction and the geomagnetic activity (Kp index) are also reported. The result of the statistical analysis is used to estimate the density, velocity, and temperature of outward flowing O+ at the high-altitude polar ionosphere. According to Meier , the velocity and temperature and the solar 0 II flux are used to calculate the emission rate factor (g-factor) for the 0 II 83.4-nm emission. We adopt the SOLAR2000 empirical solar h-radiance model [Tobiska et al., 20001 to predict the solar flux at the 0 II emission, and simulate the 0 II images taken from the lunar orbit at the position of 00 and 06 MLT, which are illustrated in Figures 1 and 2, respectively. The upper half parts of the figures
Fig. 1. The simulated 0 II imagery of the polar wind taken at the position of 00 MLT from the lunar orbit. The northern hemisphere (a) illustrates the imagery on the basis of data during the northward IMF, and the southern hemisphere (b) shows data during the southward IMF.
Fig. 2. The simulated 0 II imagery of the polar wind taken at the position of 06 MLT from the lunar orbit. The northern (a) and southern (b) hemispheres of panel illustrate the imagery on the basis of data during the northward and southward IMF, respectively.
(Figures la and 2a) show the images under the northward IMF condition and the lower half the figures (Figures lb and 2b) correspond to the southward IMF condition. The intensity is 0.8 R (Rayleigh) at maximum and about 0.1 R at 5Ra (Earth radii) far from the Earth center. It is found that the 0 II images simulated at 00 and 06 MLT under the same IMF condition are quite different even though the static-state outflow flux is given as the boundary condition. A 3-Dimensional O+ distribution under a steady condition will be obtained from the approach of tomography using some images taken at different MLT positions by the UPI-TEX imager. Instrument Design The key point for the 0 II observation is to avoid the H Ly-cl line, which is emitted from the geocorona to have the intensity of 10 kR at maximum [Rairden et al., 19861. Because the 0 II intensity from the polar wind is 0.8 R at maximum the instrument requires the high efficiency ratio of the 0 II emission to H Ly-a line of lo6 at least. We had successfully developed the instrument for the sounding rocket experiment with the efficiency ratio of 7.7~10~ [Yamazaki et al., 20021, which sufficiently rejected the H Ly-a line from the 0 II signals. The instrument is improved to the UPI-TEX imager, which has a reflective mirror, a band-pass filter, and a detector of microchannel plates (MCPs) with a position sensitive anode. The instrument parameters are summari‘zed in Table 1. The mirror has molybdenum coating, because molybdenum has a high reflectivity at the 0 II emission among the stable materials. The filter is made of a 275nm thickness indium thin film which has band-pass characteristics around the 0 II emission. And the MCPs have CsI coating to enhance the quantum efficiency. The imager has a full circular field-of-view (FOV) of 10 degrees, and the 128x128 pixels of the one image corresponding to the spatial resolution of 0.08 Ra (Earth radii) on the Earth surface from the’lunar orbit. Table 1. Design parameters of the UPI-TEX imager Mirror Filter Detector
120~mm diameter, 1.4 focal ratio Parabolic mirror 275-nm thick Iridium film MCPs with a resistive anode I$ 50 mm active diameter 12Obias angle
Pixel number FOV Resolution Mass Data bit rate
128 x 128 610” 0.078' (corresponding to O.lRs spatial resolution at Earth surface) 1.7 kg (excluding the electronics) 3 kbps (one image per 1 minute)
Distinction between Spatial Variation and Temporal Transition The imager has the total efficiency of 0.02 cps/R/pixel for the 0 II emission, on the assumption that the performances of the mirror and filter of the UPI-TEX imager are at the same level as the optical elements of XUV sensor for the previous sounding rocket, whose reflectivity and transmittance were measured to be 20% and 5% at 83.4 nm, and the quantum efficiency of MCPs is estimated to 10% from the manufacture’s catalogue. The signal count rate S of the 0 II emission and the noise count rate N of the MCPs detector are represented by the following equations,
where I is the intensity of the 0 II emission, 6 is the half angle of FOV, q is efficiency of the imager, a is the aperture of the telescope, T is the exposure time, F is the focal ratio (of 1.4 for the UPI-TEX imager), and N, is the noise count rate of MCPs per unit detection area. The measurement counts ordinarily include both the signal and the noise count rates. If these count rates follow the Gaussian distribution, the signal and noise counts is distinguished when the following equation is satisfied. S>J3TG
The S/N ratio defined by the ratio of both sides of Eq. (3) is represented as a function of the exposure time and the 0 II intensity in Figure 3. If a required condition for significant 0 II observation is that the S/N ratio is larger than 1, the UPI-TEX imager can detect the O+ outflow above the polar ionosphere with a spatial and temporal resolution of 0.1 Ra and 30 minutes at the maximum intensity. If the count rates in the 3 x 3 pixels are summed up, the temporal resolution is improved to be 3 minutes despite of the low spatial resolution of 0.3 RE.
Fig. 3. The estimated S/N ratio shown as a function of the exposure time and the 0 II intensity. If the S/N ratio needs to be larger than 1 for the significant observation, the 0 II image will be obtained a image of the area with the maximum intensity every 30 minutes.
SUMMARY We plan to observe the 0 II emission from the polar wind by the UPI-TEX imager on SELENE. The simulation shows the 0 JI intensity of 0.8 R at the maximum, and 0.1 R at the position of 5 Ra from the Earth center. If the UPI-TEX imager have the performance as the same level of the previous instrument for the sounding rocket, it can produce the image with adequate S/N ratio every 30 minutes, i.e., we can distinguish the temporal variation over 30 minutes from the spatial distribution of O.lRa. By data processing on the ground level we obtain the significant image every 3 minutes with a 0.3 Ra spatial resolution. We conclude that the UPI-TEX imager have enough performance to detect the oxygen ion outflow and the transport initial route from the polar ionosphere into the magnetosphere. ACKNOWLEDGMENTS The authors are grateful to the members of the UP1 team for their fruitful discussion to promote scientific and technological research. The authors also thank to all the members of the SELENE project team for their effort to lead the successful mission. REFERENCES Abe, T., B. A. Whalen, A. W. Yau, R. E. Horita, S. Watanabe, and E. Sagawa, EXOS D (Akebono) suprathermal mass spectrometer observations of the polar wind, J. Geophys. Res., 98, 11,19 1, 1993. Abe, T., S. Watanabe, B. A. Whalen, A. W. Yau, and E. Sagawa, Observations of polar wind and thermal ion outflow by AkebonolSMS, J. Geomag. Geoelectr., 48,319, 1996. Axford, W. I., The polar wind and the terrestrial helium budget, J. Geophys. Res., 73,6855, 1968. Banks, P. M., and T. E. Holzer, High-latitude plasma transport: The polar wind, J. Geophys. Res., 74, 6317, 1969. Burch, J. L, S. B. Mende, D. G. Mitchell, T. E. Moore, C. J. Pollock, B. W. Reinisch, B. R. Sandel, S. A. Fuselier, D. L. Gallagher, J. L. Green, J. D .Perez, and P. H. Reiff, Views of Earth’s magnetosphere with the IMAGE satellite, Science, 291, 619, 2OOla. Burch J. L, D. G. Mitchell, B. R. Sandel, P. C. Brandt, and M. Wiiest, Global dynamics of the plasmasphere and ring current during magnetic storms, Geophys. Res. Lett., 28, 1159, 2001b. Chandler, M. O., J. H. Waite Jr., and T. E. Moore, Observations of polar ion outflows, J. Geophys. Res., 96, 1421, 1991.
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Manuscript received 02 December 2002; accepted 14 February 2003